(AM-guss) (n.) a brainfart, a moment of dumb, forgetfulness; a momentary fail or hilarious mixup often leading to face/palm. An amgus can be induced by any number of things including excessive stress, homework/work, alcohol, or fatigue, or may strike randomly. Sometimes used alone as an exclamation.

Girl: "How could you forget about our date last night?!"
Guy: "Sorry! I had an amgus!"

"So Aeneas had nimbs- ah, sorry, numb limbs. Amgus..." (face/palm)

"Oh my God, I had the biggest amgus last night and couldn't remember the word for microwave!"